1. Field of the Invention
This invention relates to a turbocompressor system and a method for controlling the same, and more particularly, to such a turbocompressor system which includes a load requiring air for its operation and discharging exhaust gas, a compressor connected with the load for supplying compressed air to the load, and a turbine connected with the load and adapted to be driven by the exhaust gas discharged from the load, the turbine being operably connected with the compressor for driving thereof.
2. Description of the Prior Art
A conventional turbocompressor system of the kind described above incorporated in a fuel cell power plant is illustrated in FIG. 6.
Such a fuel cell power plant is generally high in thermal efficiency and has less influence on the environment and greater versatility in sites of installation as compared with a steam power plant employing fossile fuel such as petroleum, coal or the like. Therefore, in recent years, fuel cell power plants have been employed for special purposes such as space developments and various considerations have been made about uses of fuel cell power plants including their use as commercial power plants. Many developments of such commercial fuel cell power plants have been made in order to reduce them to practice.
In general, a fuel cell power plant system comprises a fuel cell body having electrodes of air and fuel with a layer of electrolyte interposed therebetween, a reformer for reforming a hydrocarbon fuel such as natural gas and supplying hydrogen gas as fuel to the fuel cell body, and an air-supplying means for supplying air to the air electrode of the fuel cell body and to the reformer. The performance of the fuel cell body tends to be improved as the pressures of the reaction gases increase. Consequently, the operating pressures of the respective reaction gases are commonly maintained within a range of 4 to 6 kg/cm.sup.2. In this connection, compression of air requires great power which often occupies as much as about 20 percent of the energy produced by the fuel cell body. On the other hand, reforming reaction carried out in the reformer for producing fuel gas for the fuel cell body takes place at a high temperature of about 800.degree. C. so that exhaust gas of high temperatures is discharged from the reformer. For this reason, if power for compressing air is obtained from the energy of exhaust gas from the reformer, the efficiency of the entire system will b substantially improved.
In view of the above, the conventional fuel cell power plant systems generally employ a turbocompressor as an air-supplying means. Specifically, the turbocompressor comprises a turbine adapted to be driven to run by surplus air from the air electrode of the fuel cell body and the combustion exhaust gas from the reformer, and a compressor coaxially connected with the turbine and adapted to be driven by the turbine for supplying compressed air to the fuel cell body and the reformer as required. The exhaust gas energy is thus recovered by the turbine so as to be utilized to compress air, thereby improving the efficiency of the entire fuel cell power plant system.
In such a fuel cell power plant system, wide and swift load-responsive control is required as a so-called power plant system. Particularly, it is necessary to variably control the amount of air supplied to the fuel cell body and the reformer in a wide range of, for example, from 25 to 100 percent. On the other hand, it is required from the point of view of maintaining the intended characteristics of the fuel cell body -that the- pressure of air to be supplied to the fuel cell body be maintained at a constant value even during a period of change in load in order to reduce a pressure differential between the pressure at the air electrode side and the pressure at the fuel electrode side thereby to prevent gas leakage between these electrodes or a crossover phenomenon.
In order to cope with the above-described problem, there has been proposed a fuel cell power plant system incorporating a turbocompressor system which is illustrated in FIG. 6. In FIG. 6, the turbocompressor system illustrated comprises a load section 1 including a fuel cell body (not shown) and a reformer (not shown) for reforming hydrocarbon fuel into a hydrogen enriched gas, the load section 1 thus constituting a compressed-air consuming portion in a fuel cell power plant system, and a turbocompressor 2 including a turbine 2a and a compressor 2b coaxially connected with each other through a single rotary shaft with a flow control valve 2c in the form of a variable nozzle being disposed at an inlet side of the turbine 2a. Compressed air delivered from the compressor 2b is fed to the load section 1 through an air conduit 9 from which a vent passage 11 is branched for venting part of the compressed air delivered from the compressor 2b to the ambient atmosphere. Disposed in the vent passage 11 is a vent valve 3, the opening degree of which is controlled by a computing element 6 having an electronic control circuit which is in turn controlled by a pressure controller 5 in response to the pressure of air from the compressor 2b detected by a pressure detector 4. Thus, the pressure of air from the compressor 2b is appropriately controlled by the vent valve 3 in response to the detected pressure of air from the compressor 2b. A flow-rate control valve 7 is disposed in the air conduit 9 and adapted to be controlled by a flow-rate controller 8 so as to adjust the flow rate of compressed air supplied from the compressor 2b to the load section 1 in an appropriate manner.
On the other hand, exhaust gases, discharged from the load section 1 and including combustion exhaust gas from a reformer (not shown) and surplus air from the fuel cell body (not shown), are introduced through an exhaust conduit 10 to the turbine 2a.
In order to compensate for shortage of the turbine power of the turbocompressor 2, an auxiliary burner 12 is disposed in the exhaust conduit 10. The auxiliary burner 12 is fed with compressed air from the compressor 2b through a branch conduit 16 branched from the air conduit 9, and with fuel through a fuel conduit 13. A flow-rate control valve 14 is dispose in the fuel conduit 13 for regulating the flow rate of fuel fed from a fuel source (not shown) to the auxiliary burner 12 through the fuel conduit 13, and a flow-rate control valve 17 is likewise disposed in the branch conduit 16 for regulating the flow rate of air fed to the auxiliary burner 12 through the branch conduit 16. The flow-rate control valves 14 and 17 are controlled by flow-rate controllers 15 and 18, respectively, which are in turn controlled by a pressure controller 19 associated with the pressure detector 4 in a manner such that the flow rates of fuel and air supplied to the auxiliary burner 12 are appropriately regulated in response to the pressure of compressed air from the compressor 2b detected by the pressure detector 4. The fuel and the compressed air thus properly regulated by the flow-rate control valves 14 and 17 are supplied to the auxiliary burner 12 and combusted therein, thereby affording thermal energy to the exhaust gas which is fed from the load section 1 to the turbine 2a through the exhaust conduit 10.
A bypass conduit 20 is branched from the exhaust conduit 10 at a location upstream of the flow control valve 2c on the inlet side of the turbine 2a with a bypass valve 21 being inserted in the bypass conduit 20 for regulating the flow rate of exhaust gases flowing therethrough. The bypass valve 21 is controlled by a pressure controller 23 through a computing element 24 having an electronic control circuit in response to the pressure of exhaust gases fed to the turbine 2a which is detected by a pressure detector 22. The computing element 24 acts to adjust an operating signal fed from the pressure controller 23 to the bypass valve 21 in response to the operating conditions of the turbocompressor 2, that is in response to the steady-state operating condition or the transition-state operating condition of the turbocompressor 2. The opening degree of the flow control valve 2c is controlled by a nozzle controller 25 which is in turn controlled in accordance with a load command signal issued from an appropriate load-detecting means or the like, herein shown as a load detector 26.
In operation, when the system is under steady-state operation, that is when the turbocompressor 2 is in steady-state operation, the vent valve 3 and the bypass valve 21 are held completely closed or slightly opened to certain limited degrees of opening so that the pressure controllers 5 and 23 are substantially out of operation. Such complete closing or limited opening of the vent valve 3 and the bypass valve 21 is to reduce energy loss to a minimum during the steady-state operating condition of the system. In this case, the system is in the steady-state operation and hence all the process values in the system should be intrinsically maintained constant, but in fact, the process values such as temperature, pressure or the like will gradually change due to variation in suction conditions of the compressor 2b and/or variation in the amount of heat radiation of the system resulting from change in the external temperature and/or moisture during operation of the system. It is important to maintain the pressure of air delivered from the compressor 2b at a constant value at all times irrespective of such variation in the process values, as referred to above. In this regard, the air pressure delivered from the compressor 2b is regulated by controlling the combustion of fuel in the auxiliary burner 12 by means of the pressure controller 19 through the intermediary of the flow-rate controllers 15 and 18 in a manner such that the pressure of air from the compressor 2b detected by the pressure detector 4 is made to be at a predetermined constant value. Specifically, in the steady-state operation of the system, by controlling the combustion of fuel in the auxiliary burner 12, the pressure of air delivered from the compressor 2b is regulated to fall within a specified range in a feedback manner.
Now, description will be made of the operation of the system at the time when the load on the system varies. In this case, the computing elements 6 and 24 are first controlled, prior to issuance of a load command signal by the appropriate load detector 26, so as to place the vent valve 3 under the control of the pressure controller 5 and the bypass valve 21 under the control of the pressure controller 23, respectively. Then, preset values for the flow rates of fuel and air to be fed to the auxiliary burner 12 are directly given as load command signals to the flow-rate controllers 15 and 18 so as to increase the output power of the turbine 2a. As a result, the pressure of air from the compressor 2b, being about to increase, is controlled to be constant by appropriately adjusting the opening degree of the vent valve 3 by means of the pressure controller 5. Thus, upon issuance of a load signal, the output power of the turbine 2a is increased by controlling the combustion in the auxiliary burner 12 in a feedforward manner and a part of the air from the compressor 2b thus increased is vented through the vent conduit 11 at an outlet side of the compressor 2b to the ambient atmosphere so as to adjust the pressure of air delivered from the compressor 2b at a constant value. In this manner, the output power of the turbocompressor 2 is increased to fulfill the amount of air required of the system.
When the flow rate of air discharged to the atmosphere through the vent valve 3 reaches a prescribed level, that is when the opening degree of the vent valve 3 reaches a prescribed level, the flow-rate control valve 7 in the air conduit 9 is caused to open in accordance with the requirement of the load section 1 so that an appropriate amount of air is supplied from the compressor 2b to the load section 1. In this connection, it is to be noted that on the basis of load command signals appropriately preprogrammed, the opening degree of the variable flow control valve 2c for the turbine 2a is varied in a feedforward control manner so as to cope with the changing flow rate of the exhaust gas discharged from the load section 1 to the turbine 2a. More particularly, as the opening degree of the flow-rate control valve 7 in the air conduit 9 is changed, the flow rate of exhaust gases from the system is changed in an increasing or decreasing sense, as a consequence of which the control system is preprogrammed such that the opening degree of the flow control valve 2c is so controlled as to compensate for the above change in the flow rate of the exhaust gases. In this connection, it is to be noted that in this control system, the opening degree of the flow control valve 2c is controlled in a relatively rough way and a fine control on the pressure of the exhaust gases at the inlet side of the turbine 2a is effected by adjusting the opening degree of the bypass valve 21 under the action of the pressure controller 23 through the computing element 24.
A prescribed value for the flow rate of air fed to the load section 1 is directly given, as a load command signal, to the flow-rate controller 8 so that the opening degree of the flow-rate control valve 7 is thereby adjusted in an appropriate manner, and at the same time, a prescribed value for the opening degree of the flow control valve 2c is given, as a load command signal, to the nozzle controller 25 so as to adjust the opening degree of the flow control valve 2c to the prescribed value. In this case, even if the pressure of exhaust gases at the inlet side of the turbine 2a is about to be varied for some reason, it is maintained at a constant level by appropriate adjustment of the opening degree of the bypass valve 21 due to the action of the pressure controller 23, that is by adjusting the amount of exhaust gases discharged to the outside through the bypass conduit 20.
Subsequently, when the change in the operating state of the system induced by the load command signal has been completed and the operating state of the system becomes stabilized or comes into a steady state, the delivery pressure of air from the compressor 2b is then controlled by the pressure controller 19, and the opening degrees of the vent valve 3 and the bypass valve 21 are gradually reduced to the fully closed states or certain limited degrees of opening under the action of the computing elements 6 and 24. Such operations of gradually decreasing the opening degrees of the vent valve 3 and the bypass valve 21 are to reduce energy loss of the system to a minimum, as described in the foregoing, and are effected in a finely adjusted manner so as to avoid destroying the control balance on the turbocompressor 2. During such operations, the delivery pressure of the compressor 2b is controlled to be constant by appropriately adjusting the combustion of fuel in the auxiliary burner 12 through the action of the flow-rate controllers 15 and 18. After the vent valve 3 and the bypass valve 21 have been closed completely or to the certain limited degrees of opening, the system returns to the steady-state operating condition under loading.
According to the above-described control process, the bypass valve 21 and the vent valve 3 are maintained in their fully closed or slightly opened states in the steady-state operating condition of the system, and in these states, the delivery pressure of the compressor 2b is adjusted to be constant under the combustion control of the auxiliary burner 12 so that energy loss of the system is reduced to a minimum within the range in which there is no shortage of the turbine output power. On the other hand, in cases where the load on the system changes, both the combustion of fuel in the auxiliary burner 12 and the opening degree of the flow control valve 2c for the turbine 2a are controlled in a feedforward manner in accordance with a preset program. In this case, the pressure of exhaust gases fed to the turbine 2a is controlled to be constant by means of the bypass valve 21 in a feedback manner, and at the same time, the pressure of air delivered from the compressor 2b is also controlled to be constant by means of the vent valve 3 in a feedback manner so that the instantaneously increased amounts of exhaust gases discharged from the load section 1 and air delivered from the compressor 2b are discharged to the ambient atmosphere through the bypass passage 20 and the vent passage 11. In this manner, the pressure of exhaust gases at the inlet side of the turbine 2a and the pressure of air delivered from the compressor 2b are always maintained at respective constant values in a positive manner whereby the pressures of the respective reaction gases inside the fuel cell body (the load section 1) as well as a pressure differential between the reaction gas pressures and the nitrogen gas pressure in the fuel cell body can be maintained constant at all times.
In this connection, it is to be noted that the turbine power is in direct proportion to a root of the absolute temperature T of the exhaust gases at the inlet side of the turbine 2a and the cross sectional area S of the flow control valve 2c. Accordingly, by controlling not only the combustion of fuel in the auxiliary burner 12 but also the cross sectional area (or the opening degree) of the flow control valve 2c, the output power of the turbine 2a can be increased to a desired value with the inlet temperature (or exhaust gas temperature) of the turbine 2a being suppressed to be relatively low. Therefore, the pressure of exhaust gases at the inlet side of the turbine 2a can be maintained constant at all times, and the time required for causing a load change can be shortened without difficulty.
The fuel cell power plant system employing the conventional turbocompressor system as described above, however, involves the following problems. Specifically, due to the fact that, particularly at the time of variation in load, the opening degree of the flow control valve 2c and/or the flow rate of fuel to be supplied to the auxiliary burner 12 are controlled in accordance with a preset program in a feedforward manner, it is necessary to determine beforehand varying degrees of opening of the flow control valve 2c and/or varying amounts of fuel to be supplied to the auxiliary burner 12 (referred to as the amount of auxiliary fuel hereinafter) which are considered to be most suited to a variety of loading conditions of the system. In particular, during such a period of load variation, the respective process variables of the system are changing rapidly, and thus it is difficult to determine, from moment to moment, the optimal opening degree of the variable nozzle 2c and/or the optimal flow rate of auxiliary fuel in response to the changing process values at various load-changing times. In addition, in spite of the fact that the opening degree of the flow control valve 2c has great influence on the delivery pressure of air from the compressor 2b, a computing element for determining an appropriate degree of the flow control valve 2c is required separately from means for directly controlling the compressor delivery air pressure to be at a prescribed value. Furthermore, in order to maintain the exhaust gas pressure at the inlet side of the turbine 2a substantially constant, it has been considered to adjust the opening degree of the bypass valve 21 so as to control the exhaust gas pressure at the inlet side of the turbine 2a in a feedback manner. Such a control measure, however, does not provide any good result in the event that the exhaust gas pressure at the inlet side of the turbine 2a is still low at the time when the bypass valve 21 has been fully closed. Accordingly, if the exhaust gas pressure at the inlet side of the turbine 2a is not maintained constant, the combustion of fuel in the auxiliary burner 12 and hence the output power of the turbine 12 will become unstable.